All governments or ruling parties contain competing agencies, powerful individuals or small groups that are able to commit government resources to their viewpoint on foreign policy. These configurations make up the ultimate decision unit, whose properties modify the precise character of its government's actions. The first part of this paper defines and classifies the units and conceptualizes their control variables. The second extensively describes each unit: (1) predominant leader; (2) single group; and (3) multiple autonomous groups. The third part describes how to determine the nature of the ultimate decision unit and includes a figure. The fourth part discusses how a unit shapes foreign policy behavior by describing and tabulating a preliminary empirical test of the effects that particular decision units and their associated control variables can have on a government's foreign policy actions. The concluding section reviews the postulates of the three types and points out that the paper's framework allows for comparing and contrasting the decision unit types for analysis. References are appended. (TRS)